A Study of Actuation Fatigue of Shape Memory Alloy

The use of shape memory alloy (SMA) components as actuators is an attractive option for some aerospace, automotive, and oil and gas exploration applications, especially when installation volume is limited. Fatigue failures constitute a major structural reliability issue confronting industrial entities looking to employ this technology. Knowing and understanding the failure mechanism and resulting fatigue life of SMAs is therefore necessary and useful for design and certification prior to commercial use. An in-house developed experimental set-up based on Joule heating was used to complete a comprehensive set of fatigue tests on SMA actuators composed of a nickel-rich Nickel-Titanium (Ni-Ti) alloy subjected to cyclic thermomechanical loadings. A special forced convective cooling environment has been designed that utilizes compressed air and vortex tubes to maintain the test environment at a temperature well below the martensitic finish temperature. To thermally cycle the specimen, a time controlled scheme developed using LabVIEW was used. The system design allows full thermal cycling of large SMA specimens in approximately 80 seconds. Consequently, each test (comprised of thousands of cycles) takes weeks to be completed. Actuation fatigue life results of a complete test matrix for specimens undergoing full transformation at multiple stress levels are presented. Significant spread was observed in the number of cycles to failure for specimens under same isobaric loading across different choice of applied stress. Post-mortem microscopic observation of failed specimens suggested that failure is linked to cracks initiating within Ni₃Ti precipitates. Observations reported further suggest that the failure of the specimen may not be due to plastic strain to failure.